The present invention relates to novel assay methods for determining the presence or concentration of oxalate in a sample. The present invention further relates to the cloning, sequencing and expression of formyl-CoA transferase, an enzyme used in the novel assay for the detection of oxalate.
Oxalic acid (Oxalate) is a highly toxic natural by-product of catabolism in vertebrate animals and many consumable plants. Unfortunately, a significant portion of humans are unable to properly metabolizing oxalate, a condition which may result in the formation of kidney stones in those persons. It is estimated that 70% of all kidney stones are composed of some amount of oxalate. Approximately 12 percent of the U.S. population will suffer from a kidney stone at some time in their lives, and the incidence is rising not only in the United States, but also in Sweden and Japan (Curhan, 1993). Moreover, although a healthy person breaks down or excretes sufficient quantities of oxalate to avoid excessive accumulation of oxalate in the tissues, a number of disease states are known to be associated with malfunctions of oxalate metabolism, including pyridoxine deficiency, renal failure and primary hyperoxaluria, a metabolic genetic disorder that results in the excessive deposition of oxalate in the kidneys.
Persons suffering from and at risk for developing kidney stones, as well as patients with lipid malabsorption problems (e.g., sprue, pancreatic insufficiency, inflammatory intestinal disease, bowel resection, etc.), tend to have elevated levels of urinary oxalate, a fact that has been exploited as a means for identifying individuals at risk. While elevated levels of oxalate may be present in urine, detecting elevated levels of oxalate in serum has not been routine due to the difficulty in detecting the low levels of oxalate present in serum.
Most previous methods for measuring oxalate in a biological sample first require the isolation of the oxalate by precipitation, solvent extraction, or an ion-exchange absorption (Hodgkinson, 1970). Quantitation of the isolated oxalate may be determined by any one of several methods including colorimetry, fluorometry, gas-liquid chromatography or isotope dilution techniques. Because many of the oxalate isolation techniques used in these analytical methods are not quantitative, it is normally necessary to correct for the low recovery of oxalate by adding a .sup.14 C-labeled oxalic acid internal standard, which further complicates the analytical method. All these methods are laborious, and consequently expensive because of the amount of skilled laboratory technician time which must be employed. In addition, isolation of the oxalate may require relatively large sample volumes for starting material.
Recently, several advances in the detection and quantitation of oxalate have been made through the use of (a) oxalate degrading enzymes and (b) high performance liquid chromatography. One commercially-available enzymatic test (Sigma Chemical Company, St. Louis, Mo.) employs oxalate oxidase to oxidize oxalate to carbon dioxide and hydrogen peroxide. The hydrogen peroxide produced can then be measured colorimetrically in a second enzymatic reaction in the presence of peroxidase.
In another enzymatic method for measuring oxalate, oxalate decarboxylase is used to convert oxalate to carbon dioxide and formate. The resultant carbon dioxide can be measured manometrically, by the pH change in a carbon dioxide trapping buffer or by the color change in a pH indicator buffer. Whatever method of carbon dioxide assay is adopted, the time required for diffusion and equilibration of carbon dioxide is much longer than is desirable for a rapid analytical method.
Alternatively, the formate produced by the action of oxalate decarboxylase can be assayed with formate dehydrogenase in an NAD/NADH coupled reaction, as described in Costello, 1976 and Yriberri, 1980. This method is both cumbersome and time-consuming because oxalate decarboxylase and formate dehydrogenase differ in their optimum pH requirements, thus necessitating a pH adjustment during the analysis.
Another commercially available enzymatic test (Boehringer Mannheim) cleaves oxalate to formate and carbon dioxide, then oxidizes the formate to bicarbonate by NAD in the presence of the enzyme formate dehydrogenase. The amount of NADH is determined by means of its absorbance at 334, 340, or 365 nm. Another test ("STONE RISK" by Mission Pharmacal) measures oxalate as a part of a battery of tests for kidney stones.
As illustrated above, the currently existing assays for oxalate suffer from numerous problems, including cost, inaccuracy, reliability, complexity, and lack of sensitivity. Accordingly, it is an object of the subject invention to provide a simple, accurate, and sensitive assay for the detection of low levels of oxalate in a biological sample.